Toyota Cressida/Corona/Crown/MarkII 1970-1982 Repair Guide

Catalytic Converters


All Mark II and all California models were equipped with catalytic converters in 1975. Since 1975, stricter emission level standards have made it necessary for all Toyota models to utilize one or another (or both) type(s) of catalytic converter(s).

Earlier models and today's Canadian models use an oxidation converter. That is, one that converts carbon monoxide and hydrocarbons into carbon dioxide and water.

A three-way (TWC) catalytic converter is used (by itself or with an oxidation converter) on late models. The TWC acts on all three major pollutants. Hydrocarbons and carbon monoxide are oxidized in the usual manner (into carbon dioxide and water) and the oxides of nitrogen are reduced to free oxygen and nitrogen.

An air pump is used to supply air to the exhaust system to aid in the reaction. A thermosensor, inserted into the converter, shuts off the air supply if the catalyst temperature becomes excessive.

The same sensor circuit also causes a dash warning light labeled EXH TEMP to come on when the catalyst temperature gets too high.

It is normal for the light to come on temporarily if the car is being driven downhill for long periods of time (such as descending a mountain). The light will come on and stay on if the air injection system is malfunctioning or if the engine is misfiring.


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Fig. Fig. 1 Three-Way Catalyst (TWC) system cutaway view-Cressida

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Fig. Fig. 2 Three-Way Catalyst (TWC) system operating conditions-Cressida

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Fig. Fig. 3 Three-Way Catalyst (TWC) system cutaway view-USA Corona

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Fig. Fig. 4 Three-Way Catalyst (TWC) system operating conditions-Corona

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Fig. Fig. 5 Oxidation Catalyst (OC) system cutaway view-Canadian Corona

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Fig. Fig. 6 Oxidation Catalyst (OC) system operating conditions-Canadian Corona

Use only unleaded fuel.
Avoid prolonged idling. The engine should run no longer than 20 minutes at curb idle, nor longer than 10 minutes at fast idle.
Reduce the fast idle speed, by quickly depressing and releasing the accelerator pedal, as soon as the coolant temperature reaches 120°F.
Do not disconnect any spark plug leads while the engine is running.
Make engine compression checks as quickly as possible.
Do not dispose of the catalyst in a place where anything coated with grease, gas, or oil is present. Spontaneous combustion could result.


The warning light comes on while the engine is being cranked, to test its operation, just like any of the other warning lights.

  1. If the warning light comes on and stays on, check the components of the air injection system as previously outlined. If these are not defective, check the ignition system for faulty leads, plugs, points, or control box.
  3. If no problems can be found in Step 1, check the wiring for the light for shorts or opened circuits.
  5. If nothing else can be found wrong in Steps 1 and 2, check the operation of the emission control system vacuum switching valve or computer, either by substitution of a new unit, or by taking it to a service facility which has Toyota's special emission control system checker.


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Fig. Fig. 7 When installing a catalytic converter, be sure the clamps are secure and the O-rings are in place

Do not perform this operation on a hot (or even warm) engine. Catalyst temperatures may go as high as 1,700°F, so any contact with the catalyst could cause severe burns.

  1. Disconnect the lead from the converter thermosensor.
  3. Remove the wiring shield.
  5. Unfasten the pipe clamp securing bolts at either end of the converter. Remove the clamps.
  7. Push the tailpipe rearward and remove the converter, complete with thermosensor.
  9. Carry the converter with the thermo-sensor upward to prevent the catalyst from falling out.
  11. Unfasten the screws and withdraw the thermosensor and gasket.

To install:

  1. Place a new gasket on the thermosensor. Push the thermosensor into the converter and secure it with its two bolts. Be careful not to drop the thermosensor.

Service replacement converters are provided with a plastic thermosensor guide. Slide the sensor into the guide to install it. Do not remove the guide.

  1. Install new gaskets on the converter mounting flanges.
  3. Secure the converter with its mounting clamps.
  5. If the converter is attached to the body with rubber O-rings, install the O-rings over the body and converter mounting hooks.
  7. Install the wire protector and connect the lead to the thermosensor.